6o POPULAR SCIENCE MONTHLY. 



optical theory may be deemed conclusive, possibly final, so far as the 

 general proposition is concerned that it is the science of a wave-motion. 

 In a few cases, indeed, such as the photography of the actual nodes of 

 a standing waves ystem, by Wiener, we reach the firm ground of direct 

 observation. 



Optics has nevertheless certain distinctly speculative features. 

 Wave-motion demands a medium. The enormous velocity of light ex- 

 cludes known forms of matter; the transmission of radiation in vacuo 

 and through outer space from the most remote regions of the universe, 

 and at the same time through solids such as glass demands that this 

 medium shall have properties very different from that of any sub- 

 stance with which chemistry has made us acquainted. 



The assumption of a medium is, indeed, an intellectual necessity 

 and the attempt to specify definitely the properties which it must pos- 

 sess in order to fulfill the extraordinary functions assigned to it has 

 afforded a field for the highest display of scientific acumen. While 

 the problem of the mechanism of the luminiferous ether has not as yet 

 met with a satisfactory solution, the ingenuity and imaginative power 

 developed in the attack upon its difficulties command our admiration. 



Happily the development of what may be termed the older optics 

 did not depend upon any complete formulation of the mechanics of 

 the ether. Just as the whole of the older mechanics was built up 

 from Kepler's laws, Newton's laws of motion, the law of gravitational 

 attraction, the law of inverse squares, etc., without any necessity of 

 describing the mechanics of gravitation or of any force, or of matter 

 itself, so the system of geometrical relations involved in the con- 

 sideration of reflection and refraction, diffraction, interference and 

 polarization was brought to virtual completion without introducing 

 the troublesome questions of the nature of the ether and the consti- 

 tution of matter. 



Underlying this field of geometrical optics or what I have just 

 termed the older optics are, however, a host of fundamental questions 

 of the utmost interest and importance, the treatment of which depends 

 upon molecular mechanics and the mechanics of the ether. Our 

 theories as to the nature and causes of radiation, of absorption and of 

 dispersion, for example, belong to the newer optics and are based upon 

 our conceptions of the constitution of matter; and since our ideas con- 

 cerning the nature of matter, like our knowledge of the ether, is purely 

 speculative, the science of optics has a doubly speculative basis. One 

 type of selective absorption, for example, is ascribed to resonance of 

 the particles of the absorbing substance, and our modern dispersion 

 theories depend upon the assumption of natural periods of vibration 

 of the particles of the refracting medium of the same order of fre- 

 quency as that of the light waves. When the frequency of the waves 

 falling upon a substance coincides with the natural period of vibration 

 of the particles of the latter we have selective absorption, and accom- 



